1. Field of the Invention
The present invention relates to a method of manufacturing a plastic lens which contains photochromic material. More particularly, the invention relates to depositing photochromic material on a plastic lens and then heating the lens to permeate the lens with the photochromic material.
2. Background of the Invention
Plastic lenses are widely used as optical lenses, especially as lenses used for eyeglasses due to their ease of shaping, light weight, durability against cracking and ease of coloring. Prior lenses made of plastics were generally worse than lenses made of inorganic glasses in terms of photochromic characteristics such as coloring density, coloring speed, color fading speed and durability in aperture. Conventionally, photochromic characteristics were provided to plastic lenses by the following methods: (1) premixing a powder or a pellet form of plastic lens material with photochromic materials followed by melting and mixing them together; (2) dispersing photochromic material in liquid monomers which are polymerized to form a plastic lens after polymerization; (3) immersing the hardened plastic lens material into a solvent with dispersed photochromic; (4) dissolving and dispersing the photochromic material into a liquid form of corresponding polymers and/or monomers, and solvent containing solutions if necessary, coating the surfaces of various lens materials, such as inorganic glasses and plastics, with the solutions, followed by drying and hardening the coated lens; (5) dissolving and dispersing the photochromic in a corresponding solvent, coating the surface of the plastic lens material with the photochromic solution and then heating the coating to permeate the photochromic into the plastic lens material.
However, the conventional methods listed above are not suitable for forming photochromic plastic lenses. Methods (1) and (2) cannot be applied to manufacturing plastic eyeglass lenses for correcting visual acuity. A photochromic lens formed by method (1) or (2) is hardened with the photochromic evenly dispersed inside. Since the thickness of eyeglass lenses differ from point to point, the resulting density of the photochromic will also vary in proportion to the thickness. Therefore, it is not possible to produce a lens with an even density of photochromic using method (1) or (2). Methods (3) and (5) require solvent to disperse the photochromic. However, it is difficult to use solvents such that the surface of the plastic lens material is not disturbed or degraded. When using method (4), the types of polymers and/or monomers used to coat the surface of the plastic lens materials are limited. Furthermore, method (4) has a disadvantage of low productivity when coloring since coating liquids must be prepared for each color.
When forming lenses other than photochromic lenses, such as tinted lenses, the lens material is first heated. Sublimation dyes are then evaporated near the heated surface of the lens using a resistant heater which deposits the dyes onto the surface of the lens, and permeates the dyes into the lens material. However, such a manufacturing method has a low probability of dye adhering to the lens, and is problematic since most dyes would adhere to a surrounding vacuum chamber, such as a bell chamber. Besides, this method is a slow process since deposition of sublimation dyes onto the plastic lens material and heat permeation of the materials take place in a vacuum. Such a process is not suitable for the manufacture of darker, highly tinted lenses.
In the prior photochromic lens manufacturing methods, only lenses having a color on the entire surface, which degrades over time, could be manufactured. The color then degrades over time. Furthermore, it was difficult to manufacture the photochromic lenses with two or more colors, especially with three or more colors, with existing manufacturing methods.
Due to their nature, photochromic lenses are frequently used outdoors. Therefore, photochromic lenses must be manufactured to withstand various environments. Thus, there is a need to improve durability, such as scratch resistivity, impact resistivity, adhesiveness and pollution resistivity, especially when a region of diffused photochromic is inside the surface of the plastic lens material. There is also a need for improving the light controlling speed which is characteristic of photochromic lenses. The photochromic lenses having these characteristics must be manufactured easily at low cost.
The purpose of the present invention is to answer the above mentioned problems, and to enable the manufacture of photochromic lenses at low cost and with high productivity while producing lenses which are high in durability and photochromic characteristics.